Various materials and processes have been described for use in the production of microporous films. For example, the Elton et al U.S. Pat. No. 3,870,593, incorporated herein by reference, describes a process wherein a microporous film is produced by dispersing finely divided particles of a non-hygroscopic inorganic salt such as calcium carbonate into a suitable polymer, e.g., by milling, forming a film of the filled polymer, and stretching the film to provide good porosity and water absorptive or transmissive characteristics. Microporous films are well known for use in various applications, typically where air and moisture permeabilities are desired together with liquid barrier properties.
Various technologies have attempted to improve the performance of microporous films and composite materials in which such films are employed by controlling the film pore size. For example, the Allegrezza, Jr. et al U.S. Pat. No. 4,824,568 discloses the formation of a microporous ultrafiltration membrane by precipitating a polymer from a solution of the polymer in a suitable solvent. Pore size is controlled through processing techniques and temperature. However, the materials employed in the disclosed techniques are costly and the processes of solvent extraction and drying require very low throughput. In addition, the membranes are prepared on the surface of a previously manufactured microporous support layer, and such a multistep process is inefficient.
Attempts to control pore formation in microporous films have also employed additives to film compositions. For example, the Yen et al U.S. Pat. No. 5,531,899 describes pore size control in microporous ion exchange films via the use of a porogen, an additional agent that is mixed into a polymer and subsequently removed to create pores. The solvent processing steps required to remove the porogen are expensive and inefficient. The Weimer et al U.S. Pat. No. 5,690,949 discloses a microporous film composition having viral barrier properties. Weimer et al disclose the use of a fluorinated compound to increase the wetting resistance of the microporous film and provide the barrier properties. The fluorinated compound is a costly additive and does not control the actual film pore size.
The Oka et al U.S. Pat. No. 5,830,603 discloses a porous battery separator film that may have varied porosity or pore sizes through the film thickness. The disclosed films comprise a fluororesin matrix made porous by a relatively complicated process of biaxially stretching and annealing a sintered, solvent-extracted paste extrusion of a fluoropolymer powder/liquid lubricant system, followed by hydrophilization of at least a portion of the film. The films typically have a porosity of at least 70% and pore sizes of up to 50 microns, much larger than is useful for applications where a liquid barrier is required, such as in disposable healthcare and hygienic products.
The Branham et al U.S. Pat. No. 6,261,674 discloses a breathable microlayer polymer film having 8 to 17,000 microlayers. The layers are alternately formed of first and second polymers, some of which may be rendered microporous. The first polymer is disclosed as more breathable than the second polymer. Branham et al disclose the use of non-standard extrusion equipment such as cutting and spreading layer multiplying die elements to form their films.
Many of the conventional processes for controlling pore size in microporous films involve the use of expensive additives and/or employ cumbersome processing techniques which are not suitable for large scale production. Accordingly, there is a continuing need for providing improvements in the performance of microporous polymer films and composite materials employing such films, for example by customizing or optimizing multiple properties of the microporous films, particularly while maintaining high production efficiency of such films using standard extrusion equipment and readily available raw materials.